1. Field of Invention
This invention pertains to control of electrical power transmission, and particularly to transmission of power between electrical systems.
2. Related Art and Other Considerations
Some electrical transformers, for example tap-changing transformers such as variacs, merely vary voltage. Other transformers, known as stationary phase shifting transformers, can divert power and move power through a torque angle.
Mere voltage-varying transformers and stationary phase shifting transformers may be adequate for interconnecting two electrical systems operating at the same electrical frequency, or for transmission within a utility company. However, such transformers are incapable of interfacing two electrical systems operating a differing frequency (e.g, inter-utility transfers of electricity).
There exist a number of areas in the world where interconnections between power systems require an asynchronous link. For some of these areas the power systems have different nominal frequencies (e.g , 60 Hz and 50 Hz). Even for interconnections in other systems having the same nominal frequency, there is no practical means of establishing a synchronous link having enough strength to permit stable operation in an interconnected mode.
The prevailing technology for accomplishing an asynchronous interconnection between power systems is high voltage direct current (HVDC) conversion.
FIG. 8 is a one-line diagram schematically illustrating a prior art HVDC interconnection system 820. FIG. 8 shows interconnection system 820 connecting a first or supply system 822 (shown as AC Power System #1) and a second or receiver system 824 (shown as AC Power System #2). AC Power System #1 is connected to interconnection system 820 by lines 826 for supplying, in the illustrated example, a three-phase input signal of frequency F1 (F1 being the frequency of supply system 822). Interconnection system 820 is connected by lines 828 to receiver system 824, with lines 828 carrying a three-phase output signal of frequency F2 from interconnection system 820 to receiver system 824.
HVDC interconnection system 820 of FIG. 8 includes a back-to-back DC link 830 situated between bus bars 832 and 834. Bus bar 832 is connected to supply lines 826 and to reactive compensation bus 842. Bus bar 834 is similarly connected to lines 828 and to reactive compensation bus 844.
Each side of back-to-back DC link 830 includes two transformers (e.g., transformers YY and Yxcex94 on the first system side; transformers YY and xcex94Y on the second system side) and a 12 pulse converter group. As illustrated in FIG. 8, the 12 pulse converter group for the first side of link 830 includes two six pulse converter groups 850 and 852; the 12 pulse converter group for the second side of link 830 includes two six pulse converter groups 860 and 862. As a three phase group is illustrated, each converter group includes six thyristors connected in a manner understood by the man skilled in the art. Smoothing filter 864 is connected between converter groups 850 and 860.
Also shown in FIG. 8 are reactive power supply systems 870 and 880 connected to reactive compensation buses 842 and 844, respectively. Reactive power supply system 870 includes a shunt reactor 871 connected to bus 842 by switch 872, as well as a plurality of filter branches 873A, 873B, 873C connected to bus 842 by switches 874A, 874B, and 874C, respectively. Similarly, reactive power supply system 880 includes a shunt reactor 881 connected to bus 844 by switch 882, as well as a filter branches 883A, 883B, 883C connected to bus 844 by switches 884A, 884B, and 884C, respectively. Although three such filter branches 873A-873C and 883A-883C have been illustrated, it should be understood that a greater number of filter branches may reside in each reactive power supply system 870, 880.
For any given HVDC installation, reactive power supply systems such as systems 870 and 880 are difficult to design and are expensive. Moreover, there are a large number of switched elements that have to be carefully coordinated with a given power level. Various constraints are simultaneously imposed, such as keeping harmonic performance below a requisite level (i.e., harmonic performance index) and yet maintaining reactive power between limits, all the while essentially constantly switching the filters in systems 870 and 880 as power changes. Concerning such restraints, see (for example) Larsen and Miller, xe2x80x9cSpecification of AC Filters for HVDC Systemsxe2x80x9d, IEEE TandD Conference, New Orleans, April 1989.
Thus, HVDC is complicated due e.g., to the need to closely coordinate harmonic filtering, controls, and reactive compensation. Moreover, HVDC has performance limits when the AC power system on either side has low capacity compared to the HVDC power rating. Further, HVDC undesirably requires significant space, due to the large number of high-voltage switches and filter banks.
Prior art rotary converters utilize a two-step conversion, having both a fully-rated generator and a fully-rated motor on the same shaft. Rotary converters have been utilized to convert power from AC to DC or from DC to AC. However, such rotary converters do not convert directly from AC to AC at differing frequencies. Moreover, rotary converters run continuously at one predetermined speed (at hundreds or thousands of RPMs), acting as motors that actually run themselves. Prior art rotary converters accordingly cannot address the problem of interconnecting two electrical systems that are random walking in their differing frequency distributions.
In a totally different field of technical endeavor, the literature describes a differential xe2x80x9cSelsynxe2x80x9d-type drive utilized for speed control of motors. See Puchstein, Llody, and Conrad, Alternating-Current Machines, 3rd Edition, John Wiley and Sons, Inc., New York, pp. 425-428, particularly FIG. 275 on page 428, and Kron, Equivalent Circuits of Electric Machinery, John Wiley and Sons, Inc., New York, pp. 150-163, particularly FIG. 9.5a on page 156. The literature cites the differential Selsyn drive only in the context of speed control of motors, i.e., motor speed control via relative speed adjustment between a motor and generator. Moreover, the differential Selsyn drive has a low bandwidth and makes no effort to dampen rotor oscillations.
An electrical interconnection system comprises a rotary transformer and a control system. The control system adjusts an angular position of the rotary transformer so that measured power transferred from a first electrical system to a second electrical system matches an inputted order power. The rotary transformer comprises a rotor assembly and a stator, with the control system adjusting a time integral of rotor speed over time.
The control system includes a first control unit and a second control unit. The first control unit compares the input order power to the measured power to generate a requested angular velocity signal The second control unit compares the requested angular velocity signal to a measured angular velocity signal of the rotary transformer to generate a converter drive signal, thereby controlling the angular positioning of the rotor assembly relative to the stator.
The rotary transformer comprises a rotor connected to the first electrical system and a stator connected to the second electrical system. A torque control unit or actuator rotates the rotor in response to the drive signal generated by the control system.
The bandwidth of the control system is such to dampen oscillations (natural oscillations of the rotor including its reaction to the transmission network into which it is integrated). The bandwidth of the first (slow) control unit is chosen to be below the lowest natural mode of oscillation; the bandwidth of the second (fast) control unit is chosen to be above the highest natural mode of oscillation. As used herein, the bandwidth of a control unit or control system refers to the speed of response of a closed-loop feedback unit or system.
The first and second electrical systems may have a differing electrical characteristic (e.g., frequency or phase). The controller bi-directionally operates the rotary transformer at a variable speed for transferring power from the first electrical system to the second electrical system or vise versa (i.e., from the second electrical system to the first electrical system).
In some embodiments, the torque control unit (actuator) is a motor. In such embodiments, the torque control unit may either directly drive the rotor, or interface with the rotor via a gear. In one particular embodiment, the gear is a worm gear.
In other embodiments, the torque control unit is integrated in the rotor assembly and stator of the rotary transformer. In such embodiments, the function of the torque control unit is accomplished by providing two sets of windings on both the rotor and the stator, a first set of windings on the rotor and stator having a different number of poles (e.g., 2 poles) than a second set of windings on the rotor and stator (e.g., 4 or more poles). The embodiments in which the torque control unit is integrated in the rotor assembly and stator of the rotary transformer include a squirrel cage inductor embodiment; DC-excited rotor (synchronous) embodiment; and, a wound rotor AC embodiment.
The interconnection system of the present system is utilizable in a substation for connecting asynchronous electrical systems, such as first and second power grids having differing electrical frequencies. The interconnection system of the invention not only transfers power, but can also modify power rapidly by accomplishing phase shift under load.
In the present invention, mechanical torque of the rotary transformer is controlled to achieve an ordered power transfer from stator to rotor windings. The present invention contrasts with prior art techniques which controlled power transfer from rotor to stator windings for the purpose of controlling torque applied to the load (and thereby its speed). Moreover, in the present invention, both rotor and stator windings are rated for full power transfer, whereas in prior art applications the rotor winding was rated only for a small fraction of the stator winding.
Importantly, the present invention avoids the prior art HVDC need to closely coordinate harmonic filtering, controls, and reactive compensation. The present invention also advantageously provides a one-step conversion.